This invention pertains to data communications systems, but more specifically to a data compression method and apparatus for use with telephone modems which transfer digital data over conventional public telephone lines utilizing standard digital processing.
Demands for interconnecting digital data terminals via telephone lines has spurred the development of modems. The advent of personal computers, telex and telefax machines and like digital devices (hereafter "data terminals") created this demand. Conventional modems (modulation-demodulation), such as the DPSK system described in U.S. Pat. No. 4,008,373 to Nash et al., convert digital output from the data terminal to a form suitable for analog telephone transmission, and then reconverts the telephone transmission at the receiving end back to a digital form suitable for use by the receiving data terminal. Thus each data terminal located at the respective transmitting and receiving ends of a communications link has an associated modem for transferring analog information with the telephone line.
It is known to provide systems for handling both analog voice and digital data transmissions in private branch exchanges, such as, for example, the switching system described in U.S. Pat. No. 4,578,789 to Middleton et al. There is also known a modem/voice data communications system which alternately routes analog voice and modem data over telephone lines in response to detection of some aspects of the transmitted data signals, such as described in U.S. Pat. Nos. 4,524,244 to Faggin et al., 4,660,218 to Hashimoto and 4,596,021 to Carter et al. However, no prior systems are known which operate in the conventional T1 environment and which achieve compression of modem data.
Telephone lines, however, being initially designed to cary relatively low-frequency human voice signals, do not efficiently transfer digital data bits between data terminals via modems. In most cases, data terminals are capable of handling high-speed bit transfer rates but bandwidth limitations of telephone lines, among other things, limit the bit transfer rate. Moreover, conventional T1 or like telephone networks do not include means for determining the type of originating data, e.g., whether from voice or data terminal, so its transmission technique does not "adapt" to a mode efficiently suited for the type of originating data.
Modems employ a variety of techniques for interfacing data terminals and telephone lines. Techniques such as tone encoding (acoustic couplers) and frequency shift keying (FSK) of a carrier tone are presently used to encode data terminal outputs. In one widely known technique known as the Federal Standard issued by the U.S. General Service Administration, unique dibit pairs emanating from the data terminal are encoded by unique phase changes between signalling elements (e.g., segments) of a low-frequency (1800 hz) sine wave. One Federal Standard specification, for example, provides for a 45.degree. phase change between a contiguous pair of signalling elements to represent the dibit "00", 135.degree. phase change to represent dibit "01", 225.degree. phase change to represent dibit "11", and 315.degree. phase change to represent dibit "10". It achieves bit rate transmission of 2400 bits/second at 1200 bauds. The phase change is defined as the actual phase shift in the transition region between successive signalling elements or bauds and, in this example, one baud equals one and one-half cycles of the tone carrier to attain transmission of 1200 bauds/second. Under a different CCITT standard, a V.27 protocol specifies 8-phase differential encoding characterized by phase changes of 0.degree., 45.degree., 90.degree., 135.degree., 180.degree. , 225.degree., 270.degree. and 315.degree. between contiguous signalling elements of an 1800 hz tone carrier to represent respectively tribit values of "001", "000", "010", "011", "111", "110", "100", and "101" in order to achieve a 4800 bits/second bit rate. In this case, the 1800 Hz tone carrier is modulated at 1600 bauds/second to achieve 4800 bits/second transmission.
In each case, the receiving modem reconstructs the dibit pairs or tribit values by detecting respective phase shifts between signalling element. Modem modulation protocols provide various bit rates from 1200 to 9600 bits/second where the higher bit rates require smaller increments of phase encoding and detection. Of course, smaller incremental phase differentials necessarily involve more complex encoding and detecting techniques, and for the most part, an increased risk of data error. To correct probable errors, many modems utilize an error correction encoding and recovery techniques. Thus the actual bit rates in modem transfers incur some error detection and recovery overhead.
Because present telephone networks employ the well known T1 transmission protocols, substantial inefficiencies result when conveying digital data between data terminals. Under the T1 protocol, an analog voice signal at the transmitting end is sampled and converted to an 8-bit data byte (actually a 14-bit quantizing level which is then compressed under .mu.law compression to 8 bits) using analog-to-digital converters. The sampling rate is 8 kHz, and at the receiving end, successive data bytes are reconverted to an analog signal by a reverse algorithm using digital-to-analog converters. The 1800 Hz modem tone carrier conveying information from the data terminal undergoes the same T1 processing in the telephone network. Thus, the use of modems in a T1 network subjects data terminal outputs to D/A conversion in the transmitting modem, A/D and a subsequent D/A conversion in the T1 telephone network, and yet another A/D conversion in the receiving modem.
Under the Federal Standard discussed above, each baud (e.g. a signalling element of 1.5 cycles) requires about 53.33 bits ((8000 hz/1800 hz) 1.5 cycles/dibit.multidot.8 bits/sample) to transfer one dibit pair over the T1 network, that is, 6.66 8-bit samples per signalling element. Under the 1600 baud CCITT standard, the T1 network produces five 8-bit samples per signalling element for a total transmission of forty bits to represent one tribit value. Substantial waste occurs because of the superfluous conversions and reconversions. According to the present invention, I provide modem compression by utilizing a tone carrier encoding technique particularly adapted to the T1 or like protocol to maximize bit rate transmission between data terminals. I also provide a means to detect the presence of modem data (e.g. tone carrier) in the T1 or like environment so that the telephone network may adaptively switch to its modem compression system, on call. At least one prior system described in U.S. Pat. No. 3,943,285 to Ragsdale et al. is known to achieve some bit savings in modem transmissions utilizing a modem multiplexing technique, but not within a T1 or like protocol.
Accordingly, it is a general objective of the present invention to provide means for increasing data transfer rates of data terminals utilizing modems for communicating through a telephone network utilizing T1 or like transmission protocols.
It is another objective to the present invention to provide means by which a T1 or like telephone network can adapt to a transmission mode best suited for the type of originating data, e.g., analog speech data or digital data from data terminal or computer.
In view of inefficiencies inherent in telephone networks in handling modem data in a T1 telephone transmission network, it is a more specific objective of the present invention to provide a modem data compression technique and apparatus for compressing modem data.
It is another specific objective of the present invention to provide a digital data compression method and apparatus particularly adapted for use with telephone transmission protocols which sample and digitize analog signals during the transmission of information.
In meeting the foregoing objectives, I was confronted with the problem of providing means to accomplish encoding and decoding, e.g., compression and decompression, within the time interval between receipt of 8-bit data samples transmitted over the T1 network and without delay. In the T1 system where typically twenty-four calls (thirty calls in Europe) are multiplexed over one telephone line, these 8-bit T1 data sample appear every 125 .mu.seconds upon sampling the analog sine wave at 8 kHz. Backlogs in processing these data samples must be avoided to reduce required memory space which, if accumulating, renders the system impracticable.